(1) Field of the Invention
The present invention relates to a copolyester type hot-melt adhesive. More particularly, the present invention relates to a copolyester type hot-melt adhesive suitable for manufacturing metal vessels having a circumferential side seam by bonding circumferential open ends of cup-shaped lower and upper members to each other.
(2) Description of the Prior Art
A bottle-shaped metal vessel having a circumferential side seam formed by lap-bonding circumferential open ends of cup-shaped upper and lower members obtained by draw forming or draw-ironing forming of a metal blank is advantageous in various points over a so-called can-shaped metal vessel.
As conventional packing metal vessels, there can be mentioned so-called three-piece cans in which can lids are double-seamed to the top and bottom of a can body having a side seam to form sealed portions, and so-called two-piece cans in which a can lid is double-seamed to an open end of a cup-shaped can body formed by drawing or draw-ironing of a metal blank.
However, these metal vessels having a double-seamed structure are insufficient in the pressure resistance of the sealed portion. Furthermore, these metal vessels involve a problem in connection with the saving of the material of the metal blank. More specifically, in the same formed by double seaming, the material constituting the seam is first deformed by the load imposed on the seam, and leakage on the seam or fracture of the seam is caused under a relatively small load by this deformation. In order to prevent this disadvantage, it is necessary that the thickness of the metal blank should considerably be increased. In the field of packing vessels, from the economical viewpoint and in order to reduce the weight of the vessel, it always is required to reduce the thickness of the metal blank. In the case where the thickness of the wall of the vessel barrel is reduced, buckling is readily caused by a load applied in the axial direction of the vessel at the double seaming step or the flanging or other preparing step.
In case of a bottle-shaped metal vessel formed by lap-bonding open ends of cup-shaped upper and lower members, even if the thickness of the metal blank is extremely small, no deformation of the material constituting the seam is caused and the seam can resist up to a load corresponding to the shear strength of the seam irrespectively of the thickness of the metal blank. Furthermore, since the double seaming step is unnecessary, this metal vessel is advantageous in that the thickness of the side wall of the vessel can be reduced without the risk of buckling.
However, when a circumferential side seam is formed by lap-bonding open ends of cup-shaped lower and upper members, various limitations are imposed on adhesives used for lap bonding.
More specifically, although both the ends of a straight seam of a straight can body formed by lap bonding are mechanically secured by seaming with can lids, the above-mentioned circumferential side seam is not mechanically secured along the entire circumference at all, and the dimensional deformation of the seam per se is readily caused. Furthermore, since the diameter of the open end portion is going to change when the temperature changes, a stress is readily generated in the adhesive layer. Moreover, since the thickness of the open end portion constituting the seam is reduced in many cases, the seam easily undergoes deformation when an external force is applied. Therefore, it is required that the adhesive used should have a strong bonding force to the coating on the surface of the metal blank and that the adhesive layer should have high mechanical strength and dimension stability and the physical properties of the adhesive layer should be thermally stable and not be changed with the lapse of time.
In this metal vessel having a circumferential side seam, the load applied to the vessel at the heat sterilization or during the storage or transportation substantially acts as the shearing force on the adhesive. Moreover, the adhesive layer present in the seam is inevitably influenced by the content in the vessel since the adhesive layer is perpetually contacted with the content.
From the viewpoint of the adaptability to the bonding operation or the adhesion strength, a hot-melt adhesive has been widely used as an adhesive for the manufacture of metal vessels. Hot-melt adhesives for providing a high bonding strength are roughly divided into a polyamide type adhesive and a copolyester type adhesive. The former polyamide type adhesive is defective in that water is absorbed in the adhesive layer which is contacted with a liquid content and mechanical properties such as the creep resistance are degraded by this water absorption. As disclosed in U.S. Pat. No. 3,515,628 and Japanese Patent Application Laid-Open Specification No 88939/79, various copolyester type adhesives differing in the composition are known as the latter copolyester type adhesive. However, these copolyester type adhesives are defective in one or more of the following points:
(a) Since the heat of fusion is large at the bonding step or the temperature dependence of the viscosity is not sharp, the adaptability to the bonding operation is poor.
(b) The elongation of the adhesive in the form of a film is excessively large and the dimension stability is poor, and it is difficult to apply the adhesive to a portion to be bonded with a precise dimension without distortion.
(c) The adhesive is poor in the creep resistance, especially at a high temperature or when a content having a perpetual inner pressure, such as beer or carbonated drink, is stored for a long time, and therefore, the dimension of the seam is readily changed, resulting in reduction of the inner pressure and fracture of the seal.
(d) Contraction is readily caused by crystallization after the heat bonding, whereby the dimension of the seam is readily changed or voids or cracks are readily formed in the seam.